\documentclass{article}

\usepackage{pgfplots}
\usepgfplotslibrary{fillbetween}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
]
    \addplot+[name path=A] {x};

	\addplot+[name path=B] {0};

	\addplot fill between[of=A and B];
\end{axis}

\begin{axis}[
	at={(0cm,-10cm)}
]
    \addplot+[name path=A] {x};

	\addplot+[name path=B] {0};

	\addplot fill between[of=A and B];
\end{axis}
\end{tikzpicture}


\end{document}

